feat: support = operator in function args

[universalmind303]

Now if functions support named args, we can use python style = assignment in the function instead of the lesser known => assignment operator

ex:

# before
select * from read_csv('some_file', delimiter => ';');

# after
select * from read_csv('some_file', delimiter = ';');

Note: the => operator still works.

[universalmind303]

so it turns out this breaks a lot of generated queries, such as the ones generated from prql. We'd likely need some way to configure this per function. Closing this for now as it's a lot more work than initially intended.